In Don’t mean to alarm you, but this is something to consider ^^^ I assume the block was hot tanked, do you know for sure that the shop replaced this plug? The 389 rockers are non adjustable as built. You could remove the rocker nuts and substitute SBC nuts, those are crimped to hold the adjustment. OEM Pontiac nuts will not hold adjustment, they are plain nuts and that is why you must tighten to 20 ft. lbs. torque. If you do go to Chevy nuts then with the In and Ex lifters in the base circle run the nuts down until you remove all slack at the push rod, you only want to eliminate the vertical lash. Once you have done that you can go 1/2 turn tighter, that should be sufficient preload on the lifter. Some might suggest Poly-Locs, tall nuts with an Allen set screw to hold the adjustment. If you try those your stock valve covers will not fit.